Practical Power Plant Engineering. Zark Bedalov
the other breakers in the assembly.The breaker test position is a half‐drawn‐out position. In this position, one can fully test breaker in all aspects of control and interlocks, but without affecting the other parts of switchgear assembly.In the connected position, the breaker can be fully tested provided the incoming and the tie breakers are locked and held in a withdrawn position. This test position is very useful in the commissioning (energized) phase of the plant testing that follows the precommissioning.Similar precommissioning activities are carried out on MCCs for each motor or feeder circuit to enable the assembly to be energized and to power motors and feeders for further tests. Each motor is being bumped for its rotation to match that of the pumps or conveyor travels, etc. For this activity, the motors are decoupled from the pumps.Furthermore, the motor branch circuit breakers are also pretested to establish their minimum instantaneous protection settings to suit the motor inrush currents (see Chapter 3 for details).
Wiring: During the precommissioning, a lot of simulation will be required to be performed to test the equipment and cable wiring. This includes jumpering the contacts and injecting volts or currents from other sources to command the operation of the switchgear breakers or MCC starters. All the wiring and schematics of the field devices and hard wired safety interlocks must be verified.Wiring diagrams used to be checked during the precommissioning stage too, but these diagrams are now becoming a rarity and often obsolete. As mentioned earlier, wiring diagrams have been greatly simplified by using the communication links, such as Ethernet, DeviceNet, Modbus. The present schematics have all the terminals marked just the same as the wiring diagrams used earlier. It seems to be a trend now. Perhaps not in the industrial projects yet, but, certainly, it is a trend in the large power plants.You may then ask, how do you make cable terminations if you do not have wiring diagrams? That is a very good question. Well, what many contractors now use are the cable tabulation lists showing the terminations from the terminals shown on the equipment A to the terminals on equipment B, but without giving any significance to each wire. The wiremen can swiftly terminate the wires as listed and let someone else think if the list was right or wrong. As a result, not all the signaling is being precommissioned. Some parts of it may be rung out, but not precommissioned to verify the interlocks. It is left to the commissioning group to test it and prove it on the equipment performance basis. Again here, this approach refers to the large power plants and not industrial projects.
Transformer oil: Transformers oil is tested for its dielectric strength several days prior to energizing. New oil should have a strength of >65 kV/cm, while older oils must demonstrate the insulating strength of >60 kV/cm. If the strength is lower than those desired, the transformer oil must be purified by the heating and filtering equipment to exhaust the moisture before energizing. Oil samples will be taken from the transformer during installation a week before energizing.
Figure 1.6 presents an actual handover chart of a power plant from construction, through precommissioning, commissioning, and reliability run (RR) to operation and ownership transfer.
The precommissioning checks on a larger piece of equipment, for instance, a large hydroelectric generator, is a relatively complex endeavor. A large number of interlocks must be simulated, much of them from the software. Some precommissioning can be done and must be done, such as unit trip logic and emergency stops and safety trips. In order to make the simulations more manageable, control functions are usually broken down into a number of sequential steps. These critical offline tests are performed before the online tests are attempted in order to minimize any unforeseen inadvertent operation. During this process, the unit is tested through a restricted logic to allow the checks on part of the logic and then proceed to the next ever larger step. The rest is left for commissioning. Since there are too many interlocks to be dealt with, there is also a fear that some of the simulation jumpers may be forgotten and left behind. Jumpered contacts left behind hide unreal bypassed conditions.
1.3.2 Commissioning
Commissioning is often called wet commissioning. It is an engineering activity that follows the precommissioning phase, often called dry commissioning. Commissioning is an engineering activity dedicated to testing the plant or part of the plant, in a fully energized state of applied electricity, pressure, heat, steam, and water with all the auxiliary systems in service.
During the days of relay logic and manual plant control, the commissioning was not that extensive. Nowadays, the plants have become fully automated and generally unmanned, having inputs from numerous field devices, start, stop, overload, ready, local/remote (Loc/Rem), breaker position, trip, analogs, etc. To diagnose the plant directly on the operator's screen rather than by pulling, simulating, and jumpering, the wires in the panels takes a lot of coordinated effort in many parts of the plant at the same time.
Figure 1.6 Plant handover procedure.
Source: Courtesy of SNC‐Lavalin.
Only a fully energized condition offers a controlled testing environment that allows all the logic elements to be properly commissioned and proven. This work in a noisy environment requires good communications between all the parties. Often, the cell phones do not work due to the noise interference. On the last project, the proper communication was only achieved after the house telephones operating on fiber optic cables were installed.
Primary injection: Commissioning of large power plants carries considerable risks and safety issues. This is because commissioning is mostly performed under the primary injection when the equipment is fully energized. The operating voltage is applied, and the currents are flowing through the cables and breakers. This flow can now be monitored on the real front panel instruments and/or relays. The flow of current (power) can be changed by loading the feeders and circuit breakers.
While most of the precommissioning and troubleshooting is performed from the schematic diagrams, just like always has been, the commissioning is performed based on the operational requirements. Commissioning as mentioned above is energizing and testing of the groups of equipment working together under active pressure of oil, water, steam, air, or electricity, with a minimum of simulation. This is the first time the equipment and the control system will face fuel, water, air, oil, and electricity. Believe me, this makes a lot of difference in the equipment behavior, particularly the instrumentation. The instrumentation finally gets to be checked for flow, speed, voltage, pressure, etc. Some of the instruments will now prove to be faulty, poorly calibrated, or incorrectly selected, or perhaps incorrectly wired or leaking.
The precommissioning of the power plant control system is mostly skipped in favor of the commissioning as mentioned earlier when the generator is energized in a controlled manner and tested.
Most of the wiring or control logic (95%) is expected to be correct, but a small part (5%) may be incorrect. The operator's screen may not indicate an error, but it will show what is not functioning or functioning incorrectly. It is not an easy task to comprehend as multiple malfunctions may be a result of a single wrong input. But which? Where to start? The logic interlock strings have to be investigated by observing the schematics, by removing the wired and software interlocks, one interlock at a time to zoom in on the possible targets or “usual suspects.”
What are the interlocks? These are the “permissives” that enable or disable another drive. This may be a case of set of pumps of which one only is allowed to run at any time. Safety Interlocks are hard wired into the electrical circuits as in Figure 1.7. Process interlocks are entered into software as shown on the right.
Sometimes, missing wiring must be added, wires reversed, corrected, removed, and jumpered to make it correct according to